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Two pressures are measured for a blood pressure reading 1  : Systolic blood pressure is a measure of blood pressure while the heart is beating. 2  Diastolic pressure is a measure of blood pressure while the heart is relaxed.
Medical Definition of Diastolic. Diastolic: Referring to the time when the heart is in a period of relaxation and dilatation (expansion). The noun for diastolic is diastole. (The final letter in diastole is pronounced as a long e as in lee.) The diastolic pressure is specifically the minimum arterial pressure during relaxation and dilatation of the ventricles of the heart when the ventricles fill with blood.